29 Critical Encounters = 7 Atma. That works out at a 24.13% drop rate (so it seems that it's a 25% drop rate for me from CEs).
(Edit: Now 34 CE = 8 Atma = 23.52%)
12 OC FATEs = 0 Atma
I figured why not collate our anecdotal data. You can find Critical Encounter or FATE data for OC in your achievements under Field Operations. Non-OC FATEs have an achievement but that may be hard to decipher from the ones you've done prior to OC.
CE Data:
21.51% (17 Atma / 79 CEs * 100)
FATE Data:
4.71% (5 Atma / 106 FATEs * 100)
6.84% (5 Atma / 73 FATEs * 100)
Mixed Data:
4.37% (7 Atma / (57 CEs + 103 FATEs) * 100)
7.63% (20 Atma / (97 CEs + 165 FATEs) * 100)
8.33% (6 Atma / (32 CEs + 40 FATEs) * 100)
8.62% (22 Atma / (93 CEs + 162 FATEs) * 100)
10% (2 Atma / (10 CEs + 10 FATEs) * 100)
12.98% (20 Atma / (74 CEs + 80 FATEs) * 100)
13.08% (14 Atma / (56 CEs + 51 FATEs) * 100)
13.26% (13 Atma / (48 CEs + 50 FATEs) * 100)
25% (7 Atma / (12 CEs + 16 FATEs) * 100)
You can certainly see that the average for most seems to be 7-13% when both are included. I'm going to keep updating my CE and FATE specific data as I level all my Phantom jobs, but so far it looks rather substantial of a difference.